Added env_model from Day of Defeat & Spirit SDK mods

[SmileyAG]

Splitted changes to several pull requests for easier comparisions.

Info

Alternative to cycler_sprite for placing decorative models, env_model was exist in Day of Defeat and Spirit-based mods (like TWHL Tower 2), so it also makes easier to port maps from that games.

FGD class:

@PointClass base(Targetname, Angles, MoveWith, RenderFields) studio() = env_model : "New alternative to cyclers" 
[
	model(studio) : "Model name"
	skin(integer) : "Skin" : 0
	body(integer) : "Body" : 0
	scale(string) : "Scale (1.0 = normal size)"

	m_iszSequence_On(string) : "Sequence when on"
	m_iAction_On(choices) : "Behaviour when on" : 0 =
	[
		0: "Freeze when sequence ends"
		1: "Loop"
		2: "Change state when sequence ends"
	]

	m_iszSequence_Off(string) : "Sequence when off"
	m_iAction_Off(choices) : "Behaviour when off" : 0 =
	[
		0: "Freeze when sequence ends"
		1: "Loop"
		2: "Change state when sequence ends"
	]

	spawnflags(flags) =
	[
		1: "Initially Off" : 0
		2: "Drop to Floor" : 0
		4: "Solid" : 0
	]
]